1. Field of the Invention
The present invention relates to a mixer for a device for selective catalytic reduction of exhaust gases from internal combustion engines. The present invention in particular relates to a mixer which serves uniform distribution of a reducing agent that is introduced into the exhaust gas stream.
2. Discussion of the Related Art
During fuel combustion in an internal combustion engine (e.g. a diesel or gasoline engine), undesirable by-products are produced, inter alia, nitrogen oxides (NOx). It is in the interest of keeping the air clean desirable to reduce the NOx content in the exhaust gas. One method known for this is so-called Selective Catalytic Reduction (abbreviated SCR). This is a technique for reducing nitrogen oxides in exhaust gases, in particular from engines. In SCR, a chemical reaction occurs on a catalyst (SCR catalyst). Metal oxide mixtures (for example, titanium dioxide, vanadium pentoxide and tungsten dioxide) or zeolites (H-zeolites), or also mixtures of metal oxides with zeolite are suitable as catalysts for SCR.
For having the catalytic reaction take place, ammonia (NH3) is required which must be supplied to the exhaust gas in a suitable form. The products of the reaction are water (H2O) and nitrogen (N2). The chemical reaction at the SCR catalyst is selective, that is to say, preferably the nitrogen oxides (NO, NO2) are reduced, while undesirable side reactions (such as the oxidation of sulfur dioxide to sulfur trioxide) are largely suppressed.
Since ammonia is under the prevailing physical conditions in an internal combustion engine present in gaseous state, it is fed to the location of selective catalytic reduction preferably indirectly in the form of an aqueous urea solution (known by the name AdBlue). It can—unlike gaseous ammonia—simply be carried along in an additional tank in the vehicle. When using an aqueous urea solution for supplying the reducing agent, one makes use of the fact that urea can in a thermolytic hydrolytic reaction be decomposed in several steps to ammonia (and carbon dioxide). For having such a decomposition reaction take place in an optimal manner, sufficiently high temperatures (preferably above 200° C., more preferably at least approx 350° C.) are required. The hydrolysis step is preferably further catalytically supported.
Further details regarding SCR and obtaining ammonia by decomposition of urea supplied in an aqueous solution as Ad Blue are described, for example, in European patent application EP 2 325 452 A1.
For the catalytic reactions to take place in an optimal manner, it is necessary to distribute the aqueous urea solution and the ammonia formed from this solution at the catalyst inlet as uniformly as possible in the exhaust gas stream and across the catalyst surface. This results in a high degree of efficiency for the NOx reduction. The distribution is strongly influenced by the design and geometry of the exhaust systems. To improve distribution (for achieving the most approximate stoichiometric uniform distribution of nitrogen oxides and ammonia at the catalyst inlet) additional mixers are generally used to enhance treatment and distribution of the mixture of exhaust gases and reducing agent supplied in the SCR catalyst.
It is a drawback of known exhaust gas and mixer systems, that solid urea or unwanted intermediates (for example, cyanuric acid, melamine or other solid deposits) are deposited—to a varying degree depending on the geometry used in the thermal decomposition of urea. In particular at the beginning of engine operation (after starting the engine), as long as the exhaust gas temperatures are still low, deposit of urea on the mixer elements (blades) of the mixer occurs because the water component of AdBlue evaporates and residues remain on the blades. The melting point of dry urea is at 132° C. The lower the exhaust gas temperatures, the more unwanted solid deposits form. The exhaust gas temperatures are additionally reduced by the injection of cold AdBlue. It is desirable to avoid such deposits. In addition, the SCR reaction itself requires very high exhaust gas temperatures. The exhaust gas temperatures of diesel engines are during start-up and coasting operations so low that NOx conversion in the SCR catalyst can not occur.